1. Field of the Invention
This invention relates to an optical recording medium having a phase change recording layer which is used as a write once medium.
2. Description of the Background
Highlight is recently focused on optical recording media capable of recording information at a high density. Typical optical recording mediums include write once media which can be recorded only once and which can not be rewritten, and rewritable media wherein repeated rewriting has been enabled.
Write once media are unrewritable media which are adapted for use in documents wherein tampering of the information recorded therein may cause a serious problem as in the case of official documents. The most widely employed write once media are those using an organic dye for the recording material. Use of an organic dye, however, is associated with the difficulty of realizing a high transfer rate since recording sensitivity is likely to be insufficient when the recording is accomplished at a high speed by increasing the linear velocity of the medium. An organic dye also has relatively steep absorption spectrum and reflection spectrum, and a careful choice of the organic dye is required so that the organic dye chosen corresponds to the recording/reading wavelength. For example, when there is a high-end format which requires use of a recording/reading beam of shorter wavelength, a problem may arise that the medium of low-end format can not be recorded/read by the recording/reading beam adopted in the high-end format. There may also arise the problem that dyes corresponding to the recording/reading of shorter wavelength are difficult to design and purchase.
On the other hand, there are rewritable recording media of phase change type wherein the medium is recorded by changing the crystalline state of the recording layer by irradiating a laser beam, and read by detecting the change in the reflectivity induced in the recording layer by such change in the crystalline state. In the phase change medium which can be rewritten by overwriting, amorphous record marks are formed by irradiating the medium with a laser beam of recording power level to melt the crystalline recording layer and quenching the molten recording layer to thereby form the amorphous record marks. In the erasure, the medium is irradiated with a laser beam of erasing power level to heat the recording layer to a temperature of not less than the crystallization temperature and less than the melting temperature followed by gradual cooling to thereby crystallize the amorphous record marks. Accordingly, the overwriting can be accomplished by irradiating a single laser beam with its intensity modulated.
The medium having a phase change recording layer can be also used as a write once medium in addition to the rewritable medium as described above. When such medium is used as a write once medium, it is necessary that the amorphous record mark formed is both unerasable and unrewritable.
In the case of the write once medium produced by using an organic dye, the recording process is associated with the decomposition of the organic dye. Accordingly, it is generally believed that laser power used in the recording should be increased by 21/2 folds when the linear velocity used in the recording is doubled. In contrast, when a phase change medium is used for the write once medium, the only requirement is that the part of the medium irradiated with the recording laser beam reaches its melting point. Since the recording layer almost instantaneously absorbs the laser beam and reaches the melting point, the power of the recording laser beam does not considerably depend on the linear velocity used in the recording. In other words, there is a merit that only slight increase in the power of the recording laser beam is required even if the linear velocity used in the recording were doubled.
In spite of such situation, there has been no useful proposal on the use of a phase change medium for the write once medium.